The airflow across the sail of a sailboat is complex, and when measuring the apparent wind speed and direction using a masthead sensor, the measurement point is in disturbed air (upwash). The current state of the art is to raise the point of measurement using a “vertical” pole. This places the measurement point typically 1.3 m above the mast head, but the airflow disturbance (accelerated, turned air) is still significant, even at this height. The measurement errors, even when using a vertical pole, are typically 15% in terms of airspeed, and 7 degrees in terms of wind angle measurement which is doubled to 14 degrees when computing a wind direction (the same error compounds tack to tack). The change in the measured wind direction due to tacking is sometimes referred to as “wiggling”.
Three main sources of errors making the measurement of wind direction and/or speed more difficult are identifiable.
FIG. 1 shows schematically the wind behaviour around a sail 1 of a boat 4 as wind field lines 3. The wind measurement point 2 is typically located in front of the sail 1, in upwash. This results in error that is frequently called “upwash error”.
FIG. 2 shows an exemplary graph of the magnitude of the error in apparent wind angle (AWA) for a plurality of wind speeds. It can be seen that the error is at its maximum when the boat is heading in oblique direction with respect to the wind. This is the source of a so called “heel error”.
Furthermore, leeway creates a measurement airflow disturbance which is in the opposite sense to the airflow error. In addition, heel creates geometrical reading errors as follows (horizontal axis is apparent wind angle, vertical axis is wind speed in knots). Leeway is the angle of attack of the keel—a sideways slip is needed for the keel to create a windward lifting force. The “leeway error” is illustrated in FIG. 3 in which line A1 represents the true wind direction, A2 represents boat speed, A3 the leeway error, A4 the apparent wind speed and A5 an imaginary apparent wind direction if leeway effect did not exist.
In summary, the wind angle and speed measured by a masthead wind unit is subject to error caused by aerodynamic effects on the sails, and by heel & leeway which affect the measurement geometry of the system. The size of the error is variable, it depends on many factors: the boat characteristics, wind speed, wind angle, air temperature, humidity, etc. If not corrected, these errors give rise to problems when sailing, typically a false shift in the wind direction reading when tacking or jibing and a false change in the wind speed reading when changing from an upwind to a downwind course.
Traditionally these errors have been corrected by a complex and time consuming calibration process. The existing art attempts to compensate for these errors typically takes the form of a user defined correction tables. The tables typically include separate correction values for upwind, reaching, and downwind wind angles and/or speeds and for different true wind speeds. The problem is that the correction table is complex for a user to understand and optimize, therefore most users fail to enter or manage the correction. Furthermore, each boat has its unique variant of the above table; therefore a standard table is not supplied or not applicable. Thus, in order for the table-based correction to be possible, the boat or sail manufacturer must provide the correction data in exactly the right format for the monitoring device used or the user must perform his/her own calibration tests, which is a very demanding task. This leaves most users in a position where their displayed wind direction can change by 14 degrees when they tack the boat, and the displayed wind speed can appear to accelerate by 10% when they go from upwind to downwind.
Another approach to correction is to use a mathematical formula to model the causes/effects and ask the user to fine tune the parameters. These correction methods are, however, complex for a user to understand and adjust, therefore most users fail to enter, manage or apply corrections, and have to live with sizable measurement errors in spite of having sophisticated instrument systems on board. Furthermore, each boat has its unique variant of the above table; therefore a standard table is not supplied or is not applicable. This leaves most users in a position where their displayed wind direction can change by 14 degrees when they tack the boat, and the displayed wind speed can appear to accelerate by 10% when they go from upwind to downwind. For example Ockam Instruments and Brookes and Gatehouse provide commercial devices in which the above correction principles are applied.
Yet another approach is disclosed in WO 2004/99790, relating to a system adapted to determine the air flow conditions around one or more sails being equipped with a number of ultrasonic sensors that are distributed over both surfaces thereof. The ultrasonic sensors communicate the flow conditions in their vicinity to a central unit. However, the system requires a lot of instrumentation and is thus difficult and expensive to install.
Thus, improved wind speed and angle monitoring devices and methods are needed.